The present invention relates to a process for treating a gas by adsorption, of the type in which use is made of a pressure swing adsorption treatment unit, commonly referred to as a PSA unit, and in which said unit is regulated via the elution gas.
PSA units are commonly used for the separation and/or purification of feed gas, especially in the fields of producing hydrogen and carbon dioxide, of drying, of separating the constituents of air, etc.
“PSA-H2” units which produce substantially pure hydrogen are used with feed gases of varied origin, formed for example from gases resulting from steam reforming, refinery gases or coke-oven gases, or else formed from waste gases from ethylene or styrene production units, or from cryogenic hydrogen-carbon monoxide separation units.
Generally, a PSA unit consists of several adsorbers which follow, with a time lag, an operating cycle, subsequently referred to for convenience as a “PSA cycle”, which is uniformly distributed over as many phase times as there are adsorbers in operation, and which is formed from basic steps, namely the steps of:                adsorption at substantially a high pressure of the cycle;        co-current depressurization, generally from the high pressure of the cycle;        counter-current depressurization, generally from the low pressure of the cycle;        elution at substantially a low pressure of the cycle; and        repressurization, from the low pressure of the cycle to the high pressure of the cycle.        
Hereafter, the description relates to the operation of a PSA unit in the steady state, i.e. outside of transient periods during which the unit is started up or shut down, which generally correspond to special cycles set up for this purpose.
The main operating constraint of a PSA unit in the steady state consists of the degree of purity of the product. Under this operating condition, the treatment performance level of a PSA unit are then generally optimized either in order to maximize the extraction efficiency (amount of gas produced/amount of this gas present in the feed gas), or in order to minimize the energy consumed.
Obtained in this way is a nominal operating cycle of the PSA unit, which is determined directly as a function of the nominal operating conditions (flow rate of the feed gas, flow rate of the treated gas, composition of the feed gas, operating temperature of the unit, etc.).
When the operating conditions depart only slightly from the nominal conditions, it has been proposed in the past to regulate the operation of the PSA unit by adjusting one or more parameters of the nominal cycle so as to guarantee that the treatment performance levels remain above predefined minimum limits. Two regulations that come under this approach are:                “capacity” regulation, which consists in modifying the duration of the phase time of the cycle as a function of the variation in the feed gas flow rate; and        “purity control” regulation, which consists in modifying the phase time as a function of the purity of the treated gas.        
It is advisable here to define what is understood by cycle time and phase time (or more simply phase).
As has been described above, an adsorber will therefore begin an adsorption period until it is loaded with the constituent(s) to be stopped at the high pressure, then will be regenerated by depressurization and extraction of the adsorbed compounds before being restored in order to restart a new adsorption period. The adsorber has then carried out a “pressure cycle” and the very principle of the PSA process is to link these cycles together one after the other; it is therefore a cyclic process. The time that an adsorber takes to return to its initial state is referred to as the cycle time. In principle, each adsorber follows the same cycle with a time lag that is referred to as the phase time or more simply the phase. The following relationship therefore exists:
Phase time=cycle time/number of adsorbers and it is seen that the number of phases is equal to the number of adsorbers.
There may be any number N of adsorbers, but generally N is between 2 and 32, more typically between 4 and 16.
Ultimately, under stabilized operation, assumed here to be at reduced feed flow rate, a production having the required purity and the nominal efficiency of the cycle used is obtained.
Nevertheless, the fact remains that the unit operating under these new conditions is overdesigned with respect to a design which would have been made specifically regarding this case. The amortization of the initial investment comes down to a lower hydrogen production and the specific cost is therefore higher. Over long operating periods of the unit, this operation is economically poor.